It is desirable, for vehicle strengthening members, to maximize impact energy absorption and bending resistance while minimizing mass per unit length of the strengthening member.
When a compressive force is exerted on a strengthening member, for example, a force due to a front impact load on a vehicle's front rail or other strengthening member in the engine compartment, the strengthening member can crush in a longitudinal direction to absorb the energy of the collision. In addition, when a bending force is exerted on a strengthening member, for example a force due to a side impact load on a vehicle's front side sill, B-pillar, or other strengthening member, the strengthening member can bend to absorb the energy of the collision.
Strengthening members have traditionally had rectangular cross sections, with an axis of maximum moment of inertia aligned with a horizontal Y axis (i.e., a lateral axis) of the member. During axial crush, however, rectangular strengthening members may be susceptible to inboard and/or outboard bending (i.e., bending in the Y plane respectively toward and/or away from a vehicle centerline) about the axis of minimum moment of inertia (i.e., the vertical axis), thereby essentially providing unilateral resistance to bending. Consequently, to absorb crush energy more efficiently, a number of non-traditional strengthening member cross sections have been developed.
U.S. Pat. No. 6,588,830, for example, discloses a strengthening member having a polygonal cross section of more than four sides, resulting in greater reliability and higher energy absorbing efficiency. A strengthening member with a basic octagonal cross section is disclosed as a preferred embodiment.
U.S. Pat. No. 6,752,451 discloses a strengthening member having concave portions at the four corners of the basic rectangular cross section, resulting in four U-shaped portions forming an angle of 90 degrees with each other. To avoid cracks at the concave portions at the four corners and to increase strength, the concave portions have increased thickness and hardness. Increased thickness and hardness of the corner portions is disclosed to be achievable only by drawing or hydroforming, and therefore decreases manufacturing feasibility while increasing the mass per unit length of the strengthening member.
U.S. Pat. No. 6,752,451 makes reference to Japanese Unexamined Patent Publication No. H8-337183, which also discloses a strengthening member having concave portions at the four corners of a basic rectangular cross section, resulting in four U-shaped portions forming an angle of 90 degrees with each other. U.S. Pat. No. 6,752,451 states that its thickened concave portions provide improved crush resistance and flexural strength over H8-337183.
It may be desirable, therefore, to provide a variety of tunable strengthening member cross sections configured to achieve strength increases (i.e., load carrying and energy absorption) over basic polygonal designs, while also allowing flexibility in design to meet a range of vehicle applications. It also may be desirable to provide strengthening member configurations which achieve similar if not greater strength increases than members with thickened corners, while minimizing the mass per unit length of a member and maintaining manufacturing feasibility.
It may further be desirable to provide strengthening members that can achieve increased energy absorption and a more stable axial collapse when forces such as front and side impact forces are exerted on the member, while also conserving mass to reduce vehicle weights and meet emission requirements.